1. Field of the Invention
The present invention relates to footwear having a sole with a compressible element in a lateral heel area. More particularly, the present invention is directed toward a sole having a compressible support element designed to limit the rate at which a wearer's foot pronates.
2. Description of Background Art
Sole design for modern athletic footwear is generally characterized by a multi-layer construction comprised of an outsole, midsole, and insole. The midsole, typically a soft, foam material, attenuates impact forces generated by contact of the footwear with the ground during athletic activities. Other prior art midsoles use fluid-filled bladders of the type disclosed in U.S. Pat. Nos. 4,183,156 and 4,219,945 to Marion F. Rudy. Although foam materials succeed in providing cushioning for the foot, foam materials may also impart instability that increases in proportion to midsole thickness. For this reason, design of footwear with conventional foam midsoles involves balancing the relative degrees of cushioning and stability.
The typical motion of the foot during running proceeds as follows: First, the heel strikes the ground, followed by the ball of the foot. As the heel leaves the ground, the foot rolls forward so that the toes make contact, and finally the entire foot leaves the ground to begin another cycle. During the time that the foot is in contact with the ground and rolling forward, it also rolls from the outside or lateral side to the inside or medial side, a process called pronation. That is, normally, the outside of the heel strikes first and the toes on the inside of the foot leave the ground last. While the foot is air borne and preparing for another cycle the opposite process, called supination, occurs. Pronation, the inward roll of the foot while in contact with the ground, although normal, can be a potential source of foot and leg injury, particularly if it is excessive. The use of soft cushioning materials in the midsole of running shoes, while providing protection against impact forces, can encourage instability of the sub-talar joint of the ankle, thereby contributing to the tendency for over-pronation. This instability has been cited as a contributor to “runners knee” and other athletic injuries.
Various methods for resisting excessive pronation or instability of the sub-talar joint have been proposed and incorporated into prior art athletic shoes as “stability” devices. In general, these devices have been fashioned by modifying conventional shoe components, such as the heel counter, by modifying the midsole cushioning materials or adding a pronation control device to a midsole. Examples of these techniques are found in U.S. Pat. Nos. 4,288,929; 4,354,318; 4,255,877; 4,287,675; 4,364,188; 4,364,189; 4,297,797; 4,445,283; and 5,247,742.
One particular method of resisting over pronation, disclosed in U.S. Pat. Nos. 5,425,184; 5,625,964; and 6,055,746, all to Lyden et al. and hereby incorporated by reference, utilizes a strike zone located in the rear, lateral corner of the sole. The strike zone is segmented from the remaining heel area by a line of flexion which permits articulation of the strikezone during initial contact with the ground. The strikezone includes a portion of a fluid-filled bladder structure with a lower pressure than portions in other areas of the sole. Accordingly, the strikezone operates to limit the rate of pronation following heel strike.
U.S. Pat. Nos. 5,353,523 and 5,343,639 to Kilgore et al., hereby incorporated by reference, disclose a prior art athletic shoe wherein a portion of the foam midsole is replaced with foam columns placed between a rigid top and bottom plate. A similar, prior art article of footwear, commercially manufactured and distributed by NIKE, Inc. under the SHOX trademark, is depicted as shoe 10 in FIGS. 1 and 2. Shoe 10 includes a conventional upper 12 attached in a conventional manner to a sole 14. Sole 14 includes a midsole 18 and a conventional outsole layer 20 formed of a wear-resistant material such as a carbon-black rubber compound. Midsole 18 includes a cushioning layer (not shown) made of a conventional cushioning material such as ethyl vinyl acetate or polyurethane foam, a top plate 28, a bottom plate 30, four compliant elastomeric support elements 32 disposed between top plate 28 and bottom plate 30, and a midfoot wedge 40.
Elements 32 have the shape of hollow, cylindrical columns with integral rings circumscribing the exterior surface. Whereas the front two elements 32 have a generally horizontal lower surface, the rear two elements 32 have an upward bevel in a longitudinal direction relative to shoe 10. In combination with a corresponding bevel in outsole layer 20, the rear portion of shoe 10 includes an upward bevel that extends across the rear portion of the footwear.
Elements 32 have a beneficial effect with respect to the control of pronation. As noted, the foot typically contacts the ground in the rear-lateral corner. The foot then rolls forward and rotates from the lateral side to the medial side while in contact with the ground. When the foot initially contacts the ground, the rear-lateral support element bears the majority of the impact force associated with ground contact and deflects accordingly. As the foot rolls forward and to the medial side, the force of impact is transferred to the front-lateral support element and the rear-medial support element. At this point, the front-lateral and the rear-medial support elements are both absorbing the impact forces previously supported by only the rear-lateral support element. Accordingly, the increased resistance to compression slows the rate of rotation to the medial side, thereby countering over pronation. As the foot continues to roll forward, the front-medial support element further limits pronatory motion.
Although the design of the design of shoe 10 has a beneficial effect upon pronation, individuals with a tendency to over pronate may require an article of footwear that controls pronation to a greater degree. The present invention provides such an article of footwear.